The present invention relates generally to spray coating and more particularly relates to methods and apparatus for in-situ measurement of stress and modulus during coating.
In spray coating operations, residual stress is important for the integrity of the deposit-substrate system as well as its performance. If the magnitude of the residual stress is too high, the coating may crack, delaminate from the substrate, cause substrate warpage and the like. In service, the existing residual stress superposes with the applied stress (coming from high temperature excursions, contact with other bodies, etc.) and if the resulting stress exceeds a maximum allowable limit, failure may occur or fatigue life may be shortened. If the residual stress in the system has such a magnitude and distribution that it reduces the effects of in-service stress, it will have a beneficial effect on the component life. Therefore, it is desirable to control the stress and to understand its generation. With this understanding, process modifications can be undertaken to achieve desired properties, not only by trial-and-error but by applying the knowledge of the processing phenomena.
In thermally sprayed coatings the stress has two principal origins. First, xe2x80x9cquenchingxe2x80x9d or xe2x80x9cdepositionxe2x80x9d stress, results from rapid quenching of a molten droplet upon impact on the substrate while its contraction is restricted by adherence to the substrate. This stress component is always tensile. The second stress component, xe2x80x9cthermalxe2x80x9d stress, results during cooling of the completed deposit+substrate couple from deposition temperature to ambient temperature with the stresses developing due to differences in thermal expansivities between the substrate and the coating. Depending on the sign of this difference, the so-called xe2x80x9cthermalxe2x80x9d stress can be tensile or compressive. The superposition of these two stress contributions constitutes the final residual stress. In planar systems, the stresses generally exhibit themselves by curvature of the substrate/deposit couple.
Also of significance in evaluating the efficacy of a elastic modulus of the resulting coating. The importance of the elastic modulus in this regard is two-fold. First, the magnitude of the modulus is a direct indicator of the quality of bonding between the particle layers as well as porosity. As such, the modulus has a strong influence on the performance of the coating, e.g., in applications involving wear, erosion etc. Second, the magnitude of thermal stress is, for a given temperature difference, roughly proportional to the magnitude of the coating modulus. Therefore, a variation of the modulus strongly affects the final stress.
The stresses that are incurred during coating operations and the coating modulus of the coating are important to consistent coating quality. Therefore, it would be desirable to determine these characteristics in an in-situ manner such that the coating parameters can be adapted to insure highly consistent, high quality coatings on a substrate.
It is an object of the invention to determine process induced stresses of coatings using in-situ measurements.
It is another object of the invention to determine an elastic modulus, such as Young""s modulus, of coatings using in-situ measurements.
It is a further object of the invention to provide systems and methods for determining process induced stress and elastic modulus of a coating using in-situ temperature and curvature measurements.
In accordance with a first embodiment, an apparatus for performing in-situ curvature measurement of a substrate during a deposition process is provided which includes a clamp for retaining the substrate near one end while leaving the opposite end of the substrate free. A plurality of displacement sensors are arranged in a spaced apart fashion along the length of the substrate and are directed to a surface of the substrate opposite a surface to be coated. Each sensor provides a signal to a computer corresponding to a position of the substrate relative to the sensor. The computer receives and stores data from the displacement sensors to determine a stress evolution during a deposition process and to determine a coating modulus based upon a resultant curvature of the substrate.
The apparatus can also include a temperature sensor for providing a signal to the computer indicative of the substrate temperature during a deposition process. In another embodiment, a further displacement sensor is included which is directed to the surface of the substrate being coated, such that the deposition coating thickness can be determined. When the displacement sensors are aligned such that they are directed to a common point on the substrate, an accurate differential thickness measurement can be obtained.
A method for determining residual stress on a substrate following deposition coating in accordance with the present invention includes the steps of fixing one end of the substrate; measuring the displacement of the substrate at a plurality of points along a length of the substrate during deposition coating to determine a magnitude of curvature of the substrate; using an initial estimation of coating modulus for the substrate along with the displacement measurements to determine an estimate of the residual stress on the substrate; and using the estimate of the residual stress on the substrate to refine the estimate of the coating modulus.
A further embodiment of the present method includes the step of measuring the displacement of the substrate at a plurality of points along a length of the substrate during a cooling cycle after deposition coating to determine a magnitude of curvature of the substrate and residual stress of the substrate-coating couple.
In addition to the coating modulus and residual stress, the thermal stress and quenching stress components can also be determined.
A method of determining the thickness of a deposit coating is also provided. Such a method includes the steps of measuring an initial natural frequency and/or an initial damping factor of a substrate. An expected natural frequency and damping factor for the substrate having a desired coating thickness is calculated. During coating, periodic measurements of the natural frequency and/or damping factor of the coated substrate are performed and the coating process is terminated when the measured damping factor and/or natural frequency substantially match the expected damping factor and/or natural frequency, respectively.
These and other objects, features and advantages of the invention will become apparent from the detailed description of preferred embodiments set forth below.